The present invention relates to a network employing both time division and space switching for use in telephone exchanges that employ the technique of pulse code modulation (PCM) and permit the connection between a calling subscriber and a called subscriber.
It will be recalled that the pulse code modulation technique consists of sampling the speech signals at the rate of 8 kHz and to code these samples in 8-bit words. Subsequently the codes samples are reassembled in frames of 32 time slots corresponding to PCM junctions of 32 lines. The aforementioned digital values are given only by way of example, but they are the numbers most often used to national and international requirements. The same remarks apply to any subsequently used digital data. It is known to construct switching networks for telephone exchanges by more or less complex assemblies of circuits which are called time division switches and space switches.
It is known, for example, to construct a switching network of the type TST in which a space stage S is placed between two time division switching stages T so that the overall assembly permits the desired connections.
If the network is to be free of any interval blocking the number of output links must be approximatively equal to twice the number of access links and this connection must prevail in the space stages as well as in the time division switching stages (where the links are represented by time slots). At constant speed, this requirement leads to doubling the equipment of the time division switching stages and hence to quadrupling the equipment in the space stages. This is a great drawback. It is the object of the invention to provide a novel structure of space and time division switching networks which permits obtaining a blocking rate which is negligible in terms of modern-day requirements and with only a very minor increase of the equipment used.
The switching network according to the invention includes a certain number of elementary switching circuits, each switching circuit being able to perform at the same time a space and a time division switching and without the use of any purely time division switch of purely space switch. The elementary switching circuit used in order to build this switching network is the symmetrical time division matrix (MTS), described in the U.S. Pat. appln. Ser. No. 768,632 now issued as U.S. Pat. No. 4,093,827. The MTS circuit makes it possible to switch each one of the time slots of the eight PCM input junctions MIC1-MIC8 on any one of the time slots of the eight output junctions S1-S8, thereby providing 256 routes.
Other particularities and advantages of the invention will become clear from the following description of a number of particular embodiments of a switching network which uses, as its only and exclusive elementary switching circuit, the aforementioned MTS matrices. It will be understood that this description is given by way of non-limiting example and is not intended to restrict the scope of the invention in any way. The description is made with reference to the figures of the drawings.